The present invention relates to technology effectively applied to a display control device that superimposes plural display faces to make display output, and more particularly to technology effectively used for a navigation system that displays a current position on a map.
A display system that makes a display on a liquid crystal display panel and the screen of a CRT (Cathode Ray Tube) display device generally has a frame memory for storing image data containing pixel data of one screen. The display system makes display output as follows. While pixel data of display dots is written to the frame memory by using a drawing processor or the like, a display processing circuit successively reads the pixel data from the frame memory, converts the pixel data into display signals synchronously with vertical synchronous signals of the display device, and outputs the display signals.
In some display systems, a frame memory as described above is provided for two screens so that drawing, and display output are performed through alternate switching between an area to which image data is written and an area from which image data is read for display output. By thus using the frame memory of two screens, time room is obtained between image data writing processing and display output processing, and screen flicker can be avoided which occurs as a result of the execution of display output when image data is being written.
In other some display systems having a frame memory capable of storing image data of two screens, if display contents do not need to be updated over plural display frame periods, without switching between a memory area to which image data is written and a memory area from which image data is read, display output is made by repeatedly using image data of an identical memory area. Furthermore, some display systems have an automatic switching function (automatic rendering mode) that, when it has become possible to update display contents at the termination of drawing processing for a next screen, automatically switches between a memory area to which image data is written and a memory area from which image data is read at appropriate timing. Such an automatic switching function is achieved, for example, in such a way that, when a drawing circuit fetches a predetermined command (instruction) indicating the termination of drawing processing for one screen, the drawing circuit notifies a display processing circuit of the termination of the drawing processing and the display processing circuit switches the memory area to a memory area from which image data is read in step with a synchronous signal of a display device (e.g., Non-patent Publication 1).
[Non-patent Publication 1]
“SuperH RISC engine Peripheral LSI HD64413A Q2SD User's Manual”, Electronic Devices Sales & Marketing Group Semiconductor & Integrated Circuits Hitachi, Ltd., May 2000, pages 47 to 49